Since zeolite-based solid acid catalyst has high-density acid sites on its surface and can structurally control the selectivity, it has been used as a naphtha cracking catalyst since the 1950s and is now widely used as cracking catalyst in the petrochemical industry. In addition, due to its chemical and physical properties, it is also widely used for hydrocarbon conversion and production-related reactions, i.e., a reaction of converting syngas comprised of hydrogen and carbon monoxide to light olefins and BTEX; a reaction of converting methanol to light olefins and gasoline; a reaction of reforming methane to syngas; a cracking reaction of ethane; an oligomerization reaction of light hydrocarbons, such as ethane, ethylene, propane, propylene, etc.; an isomerization reaction of hydrocarbon compounds; a reaction of converting dimethyl ether to a light olefin and BTEX; an ethylene oligomerization reaction; a reaction of converting methanol to aromatic compounds; a synthesis of monocyclic aromatic compounds or long-chain olefin compounds from syngas; etc.
In particular, the ethylene oligomerization is a reaction where oligomers, i.e., dimers, trimers, etc., are produced from ethylene at the end of the polymerization reaction. Specifically, light hydrocarbons (C1 to C5) and heavy hydrocarbons (C6 or more) can be produced. In addition, C6 to C10 aromatic hydrocarbons can be produced through a reaction of converting methanol to monocyclic aromatic compounds. Herein, the aromatic compounds may be aromatic hydrocarbons in which an alkyl group is substituted with hydrogen of benzene. The aromatic compounds, the size of which is enough to be spread and adsorbed in the zeolite pores, may be selected.
Meanwhile, zeolite-based solid acid catalysts can also be used for a process of synthesizing monocyclic aromatic compounds and long-chain olefins from syngas. Syngas is a mixed gas of carbon monoxide and hydrogen, and further, is a raw material that can conceptually synthesize all organic compounds because it contains C, H, and O. Monocyclic aromatic compounds and long-chain olefin compounds can be prepared through a dehydrogenation process using hydrocarbons prepared via the Fischer-Tropsch synthesis while adjusting the proportion of carbon monoxide, carbon dioxide, and hydrogen, which are contained in syngas. Herein, the product thereof may be BTEX, paraffins, and olefins.
Although zeolite-based catalysts have excellent catalytic reactivity in the reactions related to the conversion of carbon resources, carbon deposition (coke) occurs on the surface of the catalysts, which in turn cause inactivation of the catalysts, and as a result, there is a limitation in the commercialization of zeolite-based catalysts. A large amount of acid sites are distributed on the zeolite surface. These acid sites act as active sites for the conversion of hydrocarbons, leading to excellent reactivity of the catalysts, and at the same time, the hydrocarbon intermediates or cations produced during the reaction can easily be adsorbed. The adsorbed hydrocarbon intermediates are grown to carbon compounds containing a large number of aromatic rings through oligomerization, hydrogenation and dehydrogenation, cyclization, aromatization, etc. The grown carbon compounds cover the reaction active sites on the catalyst surface or block the nano-micrometer-sized zeolite pores, thereby to interrupt diffusion of the reactants into the zeolite pores. As a result, the diffusion and contact of the reactants to the surface of the zeolites and the active sites in the pores are intrinsically blocked, and thereby it has a problem in that the zeolite catalysts are inactivated.
In order to reduce the inactivation of the zeolite catalysts due to carbon deposition, various methods have been attempted in synthesizing the catalysts. There is a method of shortening the diffusion path in the zeolite structure and widening the surface area, thereby reducing the inactivation effect of acid sites caused by pore clogging and improving the diffusion of the reactants to the acid sites. For example, there is a method of adjusting zeolite crystals to a nano-micrometer size, a method of de-siliconization by alkali treatment, or a method of assigning a mesoporous structure to zeolites by using a structure directing agent or organic template. In such a case, inactivation of the catalysts due to pore clogging is reduced, and as a result, the lifespan of the catalysts is increased. However, there is a limitation that the increase in the diffusion and surface area due to the adjustment of the zeolite structure does not reduce the rate and amount of carbon deposition.
Another method for reducing the catalyst inactivation caused by coke formation is to reduce the amount of acid sites on the surface of zeolite catalysts, thereby reducing adsorption of hydrocarbon intermediates produced during the reaction to the acid sites. In order to achieve the same, it is required to reduce the amount of aluminum, which is the part where acid sites are expressed, by increasing the ratio of silicon to aluminum during the synthesis of zeolites; to carry out dealumination by post-synthesis treatment; or to reduce acid sites of the zeolite catalysts by ion-exchanging alkali metals. Since the intermediates produced during the conversion of hydrocarbons are adsorbed on the acid sites of the zeolite catalysts and cause inactivation, a relatively small amount of carbon deposition is made in the catalysts with reduced acid sites, and accordingly, the lifespan of the catalysts are increased. However, because the activity of the catalysts is simultaneously decreased due to the reduction of the acid sites, it is difficult to apply the catalysts in the reaction where the acidity of catalysts is required to be high.